themed pager links

This patch tackles two issues

The pager code is not properly separating presentation from content, so writing a pager theme would possibly invole a lot of code duplication.

The default pager theme will not pass the WAI level 3 guidelines

10.5 Until user agents (including assistive technologies) render adjacent links distinctly, include non-link, printable characters (surrounded by spaces) between adjacent links.

Admittedly, this is a weak requirement, but one which is quite helpful. In this case I'm constructing an unordered list of links, which is semantically correct, and I think better follows the spirit of w3c reccomendations than the original.

I think it would be more logical to use the same approach for the whole pager, but in the end decided not to change the code too much.

The patch introduces a new function

php function theme_pager_list($pager_list) {
   if( !empty($pager_list )) {
      $output = "<ul class='pager'>\n";
      foreach( $pager_list as $pager_item ) {
          $output .= "<li>".$pager_item."</li>";
      }
      $output .="</ul>\n";
   }
   return $output;
}

which is responsible for the rendering of the pager list. The only downside is the introduction of a second loop over the same data, but I think the gains flexibility outhweghts the minor loss of performance.